Hitherto, there have been many cases wherein the same inductive load needed to be driven a plurality of times within a predetermined period of time. Examples of such cases are the solenoid driving of a fuel injector in an internal combustion engine, the phase coil driving in an equivalent polyphase driving system of a step motor, and the like. In the case of a diesel engine as one of the above examples, considerable improvements in fuel injection methods have been made for measures regarding the exhaust gas and, more particularly, for a decrease in the concentration of NO.sub.x. As one of these methods, a method has been proposed in which a pilot injection is executed prior to the main injection at the time of fuel injection in an electronically controlled unit fuel injector. In this method, a predetermined quantity of fuel is pilot-injected at a predetermined time immediately before the main injection of fuel in a pressurizing process. Combustion is gently conducted, the concentration of NO.sub.x is decreased, and also the noise of the engine is reduced, by optimally controlling the timing and the quantity of the pilot injection. A conventional pilot injection-system unit injector drives a solenoid valve, which controls the fuel injection, two times consecutively in synchronization with the predetermined timings of pilot injection and main injection.
It is usually required to promptly make a load current rise or to promptly reduce the same in order to drive an inductive load, such as a solenoid valve, with good responsiveness. To this end, as a method of promptly reducing a load current, a method is generally adopted in which a component, such as a resistance, a voltage limiting element, or the like, which absorbs energy when an electric current reduces, is inserted into an electric current circulating path which includes the load, and energy stored in an inductance of the load is consumed. In addition, a method of applying a high voltage at an initial stage of driving is often adopted so as to allow the load current to rise promptly. In this case, if the application of the high voltage continues after the solenoid valve is operated, the heat which is generated in the solenoid valve or the driving circuit increases, thereby lowering the efficiency of the load. Therefore, after the solenoid valve has finished operating, the driving is usually conducted at a voltage lower than the high voltage which was used at the initial stage of driving.
For the aforesaid reasons, a solenoid valve driving device of the conventional unit fuel injector includes a step-up circuit, for obtaining the initial high voltage from a supply voltage supplied from an on-vehicle battery, and a holding current output circuit, for holding the solenoid valve at a predetermined current value after the solenoid valve has finished operating. Generally, a capacitor for storing the energy required for the initial stage of load driving as an electric charge, an inductance for storing the energy as magnetic energy, or the like is provided in the step-up circuit, and the stored energy is promptly given to the load at the initial stage of load driving. The energy required for the initial stage is energy given to the load and energy needed for the displacement of the load as an actuator. Thus, the responsiveness of the solenoid valve at the time of a pilot injection and a main injection is improved, whereby the delay of injection timing is avoided.
However, in the conventional solenoid driving device, the same solenoid valve needs to be driven two times within a short period of time, and thus the step-up circuit is required to store a predetermined quantity of energy in the capacitor or the inductance for storing energy within the short period of time to thereby step up the voltage. The shorter the period of time required for storing the energy becomes, the larger the capacity which is needed for each power electro element for storing the energy of the step-up circuit (for example, a thyristor, a transformer, or the like). However, if an element with a large capacity is used, the step-up circuit increases in size and the cost sharply increases, which makes it difficult to make a step-up circuit with plenty of capacity. Accordingly, in a conventional driving circuit, there arises a case wherein the second step-up in the voltage is delayed. In the case of the aforesaid fuel injection, the response of the solenoid valve at the time of a main injection is delayed at this time, and thereby the behavior between the pilot injection and the main injection becomes unstable. Consequently, there arises a disadvantage in that an effect on a reduction in the concentration of NO.sub.x can not be sufficiently obtained.
A case of a step motor will be explained. In order to improve responsiveness, it is usually necessary to increase the number of phases or poles. An equivalent polyphase driving system is well known in which, for example, in relation to a three-phase machine, the phases are equivalently increased to twelve phases by increasing the number of apparent phases. FIG. 22 shows a circuit diagram of an example of an exciting coil portion of an equivalent twelve-phase driving system. In FIG. 22, coils 1, 2 and 3 are provided corresponding to a first phase through a third phase, and one end of each coil is connected to the positive electrode of a power source. Between the other end of the coil 1 and the negative electrode of the power source, a series circuit of a resistance 4a and a transistor 7a and a series circuit of a resistance 4b and a transistor 7b are connected in parallel. Similarly, between the other end of the coil 2 and the negative electrode of the power source, a series circuit of a resistance 5a and a transistor 8a and a series circuit of a resistance 5b and a transistor 8b are connected in parallel, and between the other end of the coil 3 and the negative electrode of the power source, a series circuit of a resistance 6a and a transistor 9a and a series circuit of a resistance 6b and a transistor 9b are connected in parallel. The base of each respective transistor is connected to a driving device, which is not shown, and the driving device sequentially outputs an ON signal to the bases of the respective transistors to electrify the respective transistor and to sequentially send an exciting current to the respective coil 1, 2, or 3. The current value of each phase at this time is set at two ways of 1:2 by the two resistances (for example, the resistances 4a and 4b) of each phase, and the exciting sequence of each phase by the current is shown by FIG. 23. In the drawing, "1" represents the "ON" state of the respective transistor. When the exciting current values of respective phases are sequentially increased and decreased repeatedly while shifting phases, three rotor stable positions are provided among the respective phases by the composition of the magnetomotive forces of the respective phases. As a result, an equivalent twelve-phase driving is realized, and the driving frequency is improved.
At this time, each phase coil needs to be driven twice within a predetermined period of time like a phase I at the time when the sequence in FIG. 23, for example, changes from 9 to 10 and from 10 to 11. Therefore, also in the step motor of such a system, it is conceivable that a high voltage is applied at an initial stage of driving similarly to the above in order to allow the load current of a respective one of the coils 1, 2, and 3 to promptly rise.
However, in the driving device of the step motor, disadvantages arise, as described below. As the rotation of the step motor speeds up, the time interval for driving the respective phase coil becomes shorter, whereby the same phase coil needs to be driven twice within a short period of time. Thus, the step-up circuit requires an element with a large capacity, capable of storing energy within a short period of time, which causes disadvantages in that the step-up circuit increases in size and in cost.